Effect of amounts of Mn and excess Si on precipitation of 6082 aluminum alloys

The effect of the amounts of Mn and excess Si on the precipitation sequence of 6082 aluminum alloys was investigated by a micro-vickers hardness measurement and transmission electron microscope observation. Peak hardness of specimens aged at 473 K increases with increasing amount of excess Si, while Mn content does not affect their peak hardness. Addition of Mn changes the types of precipitates in the specimens. The precipitates observed in the present study are similar to those in the balanced alloy, even though these alloys contain Si in excess. This tendency becomes marked when aging temperature becomes higher than 473 K. This is because Mn reacts Si in the matrix, forming the Mn-dispersoid and then excess Si content in the matrix decreases. On the other hand, the interface between the matrix and the Mn-dispersoid acts as the sink of the quenched-in vacancies, and the formation of GP zones is suppressed. Thus, the decrease of excess Si and the suppression of the GP zones formation lead to an in-homogenous precipitation of the metastable phases in the matrix.